Field control development of electrostatic images

ABSTRACT

FOR DEVELOPING LATENT ELECTROSTATIC IMAGES ON PAPER SHEETS THERE IS PROVIDED A DEVELOPING STATION WHICH INCLUDES A TANK FOR HOLDING A BATH OF LIQUID DEVELOPER INCLUDING AN INSULATING CARRIER LIQUID WITH PIGMENT PARTICLES DISPERSED THEREIN, AND A RESTRICTED GUIDEWAY OR CHANNEL FOR LEADING THE SHEETS DOWN INTO AND UP OUT OF THE BATH. THE UPPER MEMBER, AT LEAST, OF THE GUIDEWAY IS OF CONDUCTIVE MATERIAL AND PROVIDES A FIELD CONTROL EFFECT FOR INTENSIFYING THE DEPOSIT OF PIGMENT PARTICLES IN THE IMAGE AREAS. THE FIELD CONTROL MEMBER MUST BE PLACED CLOSE TO BUT OUT OF CONTACT WITH THE IMAGED SHEET SURFACE IN ORDER TO HAVE THE DESIRED FIELD CONTROL EFFECT. IN ORDER TO REPLACE THE LIQUID DEVELOPER IN THIS RESTRICTED CHANNEL RAPIDLY ENOUGH TO PREVENT ITS BEING STRIPPED OF PIGMENT PARTICLES, THE FIELD CONTROL MEMBER IS PROVIDED WITH OPENINGS THROUGH WHICH FRESH LIQUID DEVELOPER CAN BE RAPIDLY INTRODUCED INTO THE CHANNEL, AND SPRAY MEANS AND BAFFLES ARE ARRANGED TO EFFECT THIS DEVELOPER REPLACEMENT AS EFFICIENTLY AND RAPIDLY AS POSSIBLE. MOREOVER, THE ARRANGEMENT IS SUCH THAT HYDRODYNAMIC FORCES ARE ALSO GENERATED IN THE LIQUID DEVELOPER IN THE CHANNEL TO MAINTAIN THE SHEET AGAINST A GUIDING SURFACE AND HENCE AT AN ACCURATE SPACING FROM THE FIELD CONTROL MEMBER.

March 26, 1974 J KOUBAS 3,799,791

FIELD CQN'I'ROL DEVELOPMENT OF ELECTROSTATIC IMAGES Original Filed June50, 1966 C ONDUC 77 V5 INVENTORS. JAMES A. lfoL/sAs ATTORNEY Ifrequently carried out by ot the developer.

United States Patent a 799 9 FIELD coNTnoL DEVELOPMENT OF ELECTROSTATICIMAGES James A. Kolibas, Broadview Heights, Ohio, assignorghiAddressograph-Multigraph Corporation, Cleveland,

, .1968, which is a continuation of application Ser. No. 561,784, June30, 1966, both now abandoned. This application Apr. 17, 1972, Ser. No.244,590

' I Int. Cl. C03g 13/10, 15/10 US. Cl. 117-37 LE 8 Claims 'ABS TRACT OFTHE DISCLOSURE For developing latent electrostatic images on papersheets there is provided a developing station which includes a tank forholding a bath of liquid developer inimage areas. The field controlmember must be placed close to but out of contact with the imaged sheetsurface Continuation of application Ser. No. 763,606, Sept. 30, r

in order to have the desired field control effect. In order to replacethe liquid developer in this restricted channel rapidly enough toprevent its being stripped of pigment particles, the field controlmember is provided with openings through which fresh liquid developercan be rapidly introduced into the channel, and spray means and bafllesare arranged to effect this developer replacement as efiiciently andrapidly-as possible. Moreover,'the arrangement is such that hydrodynamicforces are also generated in the liquid developer in the channel tomaintain the sheet against a guiding surface and hence at an accuratespacing from the field control member.

This application is a continuation of application Ser.

No. 763,606, filed Sept. 30, 1968, now abandoned, which is acontinuation of application Ser. No. 561,784, filed June 30, 1966, nowabandoned. v

This invention relates to the deposition of particles upon the surfaceof a moving sheet in accordance with -a latent electrostatic patternpreviously formed thereon,

and especially in situations where it is desirable to employ a liquid asthe carrier for the particles during deposition.

'- The treating of an electrostatically imaged sheet with a liquiddeveloper having electrostatically attractable particles dispersedtherein is a well-known procedure and is immersing the sheet in a bathThe present invention is based upon the principle that improveddeposition can be obtained by means of a conductive field controlmember'placed very closeto, but accurately spaced from, the surface ofthe sheet to be developed, and embodies a means for maintaining anaccurate minimum spacing relationship between such a field controlmember. and the sheet surface under the relatively unstablecircumstances represented in progressive appli- 1 3,799,791 PatentedMar. 26, 1974 cation of developing liquid to the electrostatic image ona sheet moving through a developer bath. However, in a broader sense,the invention also involves the maintenance of accurate spacing betweenthese surfaces in a kinetic electrostatic development situation whetheror not the process relates to immersion development or to some othertype.

It may be noted at this point that a conductive field control member,placed close to the photoconductive surface during development not onlyspeeds deposit but also assists in providing a uniform deposit overlarge solid areas and avoids edge effects. By edge effects is meant theelectrical field phenomenon which results in a low intensity depositfound in the centers of large charged areas, which are intended to showa solid, uniform density when developed, but instead display dark edgesshading to grey to white centers.

This invention, in the presently preferred form provides a guide channelfor a sheet to be developed and through which the sheet is fed and fromwhich it is withdrawn by feed rollers. The channel is arranged to exposethe sheet to a bath of liquid developer, and this is brought about inthe form shown by positioning the channel to dip into a bath ofdeveloper which is maintained at a predetermined depth. One membermaking up the channel is a guide member against which the sheet restsand the other member is a conductive member placed in fixed closelyspaced relation to the guide member so as to provide a field controleffect at the place where electrostatic deposit of particles isoccurring. The sheet is held against the guide member by hydrodynamicforces generated in the liquid itself so as to provide consistentaccurate spacing between the surface of the sheet and the field controlmember. Preferably these hydrodynamic forces are generated by pumping orflowing the developing liquid rapidly through orifices in the fieldcontrol member so as to hold the sheet strongly away from the fieldcontrol member and against its locating guide member as it passes mentwith a liquid developer in accordance with the present invention; and

FIG. 2 is a detail section consisting of a portion of a FIG. 1 drawn toan enlarged scale.

Referring to the drawing, there is illustrated therein a tank 10containing a bath 12 of liquid developer of the type designed for usewith electrostatic deposition procedures, and having dispersed thereinminute electroscopic particles which will deposit upon electrostaticallycharged areas of a sheet surface. A sheet 14 provided with a latentelectrostatic image on its upper surface is led down into and then upout of the bath 12 by an assembly 16 which provides a channel 18 withinwhich the sheet is confined. The assembly 16 comprises lower or guideplate 20 and an upper or field control plate or member 22 which is madeof metal or other electrically conductive material form the functionsrespectively of introducing the sheet 14 into'the channel 18 andwithdrawing it therefrom. The

squeegee operation on the sheet to remove excess liquid therefrom andreturn the same to the bath 12.

In order to maintain an appropriate liquid level in the bath 12 and tokeep the liquid developer properly mixed and agitated, there is provideda reservoir 32 which has a supply of liquid developer therein and whichhouses a pump 34 driven by a motor 36. The pump moves the liquiddeveloper through a discharge conduit 38 to a manifold 40 extendingacross the top of the bath 12 transversely of the direction of sheetmovement. The manifold has a plurality of openings along its length, oneof which is indicated at 42. These openings are downwardly directed sothat the liquid developer which'is under the pressurein the manifold 40is sprayed or flowed forcibly downwardly against the upper surface ofthe field control member 22 as indicated at 43. The field control memberis' provided with apertures 44 some of which are arranged in the path ofthe spray 43 so as to admit the spray to the channel 18 and against theupper surface of the sheet 14 from whence it escapes to the bath 12through the open sides of the channel. In the preferred arrangement,louvers or baffles 46 are struck up from the plate 22 adjacent theapertures 44 to assist in guiding the spray in'th'e desired direction.

An overflow pipe 48 determines the proper level for liquid in the bath12 and returns any excess to the reservoir 32.

'field control effect as a result of the conductive nature of the plate22, this spacing S must be quite small and must be maintained constantwith a certain degree of accuracy. A distance of about inch has beenfound about the optimum in practice with paper sheets of usual thicknessand electrical properties, and a range of spacing of about inch to A,inch is found to be the range of effective field control values. Forconvenience, spacing-within the general ranges identified may bereferred to aseifective field control spacing. As the spacing S is madesmall for effective field control, local random variation in Shecomesof. evengreater importance. For example, avariation of- 1005 inch in S'if S=% is. highly undesirable whereas if S= inch the same fluctuationin;S' would create virtually no visible differences in the developedimage.

To minimize local. random variations it is desirable to cause the sheetto conform very closely to the guide member so that the precisionattainablein the mounting-of the plates20 and 22 will be effective tominimize ,variationsin S, and this is admirably achieved according tothis invention by the hydrodynamic action of the liquid developer itselfin that the hydrodynamic pressure 'of the liquid in spray 43 is exertedagainst the surface of a sheet v18.to hold it against the surface of theguide plate 20.

The use of the hydrodynamic action of the'developing liquid to maintainthe surface of the electrostatically imaged sheet against aguide whichinsures accurate spac ing from the surface of the conductive fieldcontrol plate 22, makes it possible to' employ a field control techniquelatterpair, i.e'. rollers 28, 30, also preferably perform a" on apractical basis with very close spacing of the field I control member,and thereby produce imaging of the highest quality. It is, moreover,possible to accomplish these results in such a way that the need forcomplex -meansfor assuring proper positioning and adequately closetolerances of spacing of the sheet across its whole surface is avoided.The hydrodynamic eifect o he veloping liquid acts automatically andsimultaneously at all points on the sheet which are in the developingzone by reason of being exposed to the liquid.

In certain respects the field control aspect of the equipment assistsdevelopment, while in other respects it has been found to inhibit it. Ingeneral the closer the field control conductor is placed to thephotoconductive surface the more rapidly the electroscopic particles aredeposited on the charged areas of the sheet. When very close spacing isinvolved, however, this deposit can be so rapid that the small amount ofdeveloper liquid available in the area between the closely spacedsurfaces tends to become stripped of particles so that subsequent imageareas are starved and do not receive appropriate deposits if the sheetmoves through the bath at normal copier speeds. This effect isaggravated to some extent by the fact that changing the liquid adjacentthe sheet surface necessarily becomes more difficult the closer thesheet surface is brought to the field control conductor since there is asmaller cross sectional area available through which fresh liquid can beintroduced.

Due to the relationships stated in the foregoing paragraph it isimportant, especially when the closer spacings are used, to purposefullymove the liquid developer in a manner that insures adequate rapidreplacement to prevent the starvation phenomenon alluded to. In thepresently disclosed form of the invention, the forceful spraying of theliquid developer is at suflicient velocity, and the liquid is introducedinto the channel 18 at a number of points which are close enoughtogether so that there is fairly rapid movement of the liquid betweenthe plates 20 and 22. Accordingly, the developer liquid in the channel18 is sufficiently energized as to move parallel to the sheet surfacewith a speed suflicient to insure that any stripped developer rapidlyescapes from the channel18,

and the channel is continually flooded with freshdeveloper liquid sothat rapid exchange of fresh particle-charged developer for strippeddeveloper is assured. Thus a sheet 14 may be moved through the channelat a speed acceptable for commercial copier use without exhibitingdeficient development in the trailing portions of the image. While onlythe field control plate 22 has been referred to as conductive, it isfound beneficial in some instances toactually apply a voltage across thegap between the field control plate and the surface of the sheet, inwhich case the guide plate 20 is also made conductive and a .source ofEMF.is connected both to the plate 22 and to ;the guide plate 20. a V

g It will be understood that various configurations and structuralarrangements may be provided, and that the proper spacing andconductivity, the manifold 40 could 'serve as the field control memberif desired.

While a preferred embodiment of the invention has beendescribedandillustrated, it is to be understood that this is capable ofvariation and modification. Ac-- cordingly, the aim in the appendedclaims is to cover all such variations and modifications as may fallwithin the .true spirit of the invention.

What is claimed is: v 1."A method of developing electrostatic latentimages on sheet surfaces which comprises:

' positioning an apertured electrically conductive field control memberand a guide member in close rela- .tiohship with a 'gap between themdefining an effect tive field control spacing;

passing a sheet, whose surface has been electrostatically imaged,between said guide member and field control member with theimagedsurface towards said field control member; while passing said sheetbetween said member, flooding the portion of the gapbetween said fieldcontrol member and said sheet with an electrostatic developer fluidcontaining particles electrically attractable to areas on the surface ofthe sheet; and forcing fluid developer through an aperture in the fieldcontrol member and against the sheet to thereby generate dynamic forcesin the fluid developer entering the gap in a direction and of amagnitude to hold the sheet consistently away from the field controlmember and against the guide member during passage of the sheet betweensaid members. 2. A method of developing electrostatic latent images onsheet surfaces which comprises:

positioning an apertured electrically conductive field control memberand a guide member in close relationship with a gap bet-ween themdefining an effective field control spacing; passing a sheet, whosesurface has been electrostatically imaged, between said guide member andfield control member with the imaged surface towards said field controlmember; while passing said sheet between said members, flooding theportion of the gap between said field control member and said sheet withan electrostatic developer fluid containing particles electricallyattractable to areas on the surface of the sheet by forcing freshdeveloper fluid through an aperture in said field control member andagainst the sheet to generate dynamic forces in the fluid developerentering the gap in a direction and of a magnitude to hold the sheetconsistently away from the field control member and against the guidemember during passage of the sheet between said member; and p causingsuch fresh fluid to be projected rapidly through the aperture into thegap with a direction of motion having a substantial component parallelto the surface of the sheet so as to bring about fluid motion within thegap parallel to the surface of the sheet to continually displace fluidin the gap from which the particles have been electrostatically removedby preceding portions of the moving sheet, and to replace it with theincoming fresh fluid rapidly enough to prevent nonuniform developmentdue to stripping of the developer available to the sheet surface. 3. Adevice for developing electrostatic latent images comprising:

a guide member; anbapertured electrically conductive field controlmemmeans mounting said members in close relationship with a gaptherebetween defining an effective field control spacing; means forfeeding a sheet whose surface has been electrostatically imaged throughsaid gap with the imaged surface towards the field control member; meansfor introducing into the portion of the gap between said field controlmember and a sheet being fed through the gap an electrostatic developerfluid containing particles electrically attractable to the image on thesheet; and means for generating dynamic forces in the fluid developerentering between said sheet and field control member in a direction andof a magnitude to hold a sheet consistently away from the field controlmember and against the guide member during its passage between saidmembers, said last-named means comprising equipment for moving developerfiuid through an aperture in the field control member and against theimaged surface of the sheet.

4. Means for developing an electrophotographic sheet having a latentelectrostatic image on one surface com- P ns= a tank for containing abath of liquid developer ineluding electroscopic pigment particlesdispersed therein;

' guide means associated with said tank and defining a path for saidsheet down into and up out of said bath of liquid developer;

a conductive field control member parallelling the path of the sheetthrough the developer at the side thereof bearing the electrostaticimage and arranged sufficiently closely to the path as determined bysaid guide means to provide an eifective field control spacing withrelation to the surface of a sheet traveling in said path;

means for moving a sheet through said path; and

means for maintaining said sheet out of rubbing contact with the surfaceof the field control member as it moves through said path.

5. For developing an electrostatic image on a sheet of material,apparatus comprising:

a container for a pool of developer;

means for guiding the sheet through the pool along a predetermined pathwhich dips into the pool with the image side of the sheet facingupwardly;

a perforate metallic member in the pool juxtaposed to the upper side ofthe path and arranged sufliciently closely to the path to provide aneffective field control spacing with relation to the surface of a sheettravelling in said path; and

means for forcibly projecting developer through openings in saidperforate member with a force and direction sufficient to maintain theupper sheet surface spaced away from the perforate member on its passagethrough the pool.

6. For developing an electrostatic image on a sheet of material,apparatus comprising:

a guide member for guiding a sheet along a development path;

a perforate conductive member arranged adjacent the guide member andhaving a generally matching configuration, said members being arrangedwith a gap between them defining an efiective field control spacing, andmutually forming a channel for the sheet;

means for moving the sheet along the path through said channel with itsimaged surface faced towards said perforate conductive member;

means for forcibly projecting liquid developer through the openings insaid perforate member with a force and direction sufiicient to maintaina body of developer in said channel between the imaged surface of asheet being processed and the adjacent surface of said conductivemember, and to continually displace the said body of developer andreplace it with the incoming developer rapidly enough to avoidstarvation effects.

7. A developing device as set forth in claim 6 which further includesmeans for forcibly projecting liquid developer through the openings insaid perforate member with a force and direction sufiicient to hold thesheet spaced from said conductive member and against said other memberduring development.

8. For developing electrostatic images on a sheet of material, apparatuscomprising a container for a pool of developer, means for guiding thesheet through the pool along a predetermined path which dips into thepool with the image side of the sheet facing upwardly, upper and lowermetallic members in the pool juxtaposed to the upper and lower sides ofthe path respectively, the upper member being perforate, and means forflowing developer downwardly through the perforate member to the imageside of the sheet.

(References on following page) 7 8 References Cited 3,368,525 2/1968Sacra 117-37 L E 3,103,153 10/1963 Limbercer 117-37 L E UNITED STATESPATENTS 3,147,686 9/1964 Limberger 9s 99 7/1964 Rltlfdd 117-47 L E2,877,132 3/1959 Matthews 117-175 8/1965 Ostensen 117-37 L E 7 1271922cu dla h 117 371 1; 5 WILLIAM D. MARTIN, Primary Examiner 11 19 4 Dir s9- 2/1965 Levene SOFOCLEOUS, Asslstant Examlner 2/1965 York 118-6374/1966 Okishima 118-637 10 5/1966 0516115611 "118- 637 96-1 L Y;118-637; 3ss-10 12/1967 Hastings et a1. 117-37 L E

